The present invention relates to machine vision vehicle service systems, and in particular to an optical target assembly configured for mounting to a surface of a vehicle, such as a vehicle wheel, during a machine-vision vehicle wheel alignment procedure.
A machine-vision vehicle service system, such as a vehicle wheel alignment system like the Series 811 Wheel Alignment System utilizing the DSP 600 Series sensors, manufactured and sold by Hunter Engineering Company of Bridgeton, Mo., consists generally of a console having a computer or processing unit, one or more display devices such as a monitor, and one or more input devices such as a keyboard. In a machine-vision vehicle wheel alignment system, one or more imaging sensor arrays are mounted away from a vehicle undergoing an alignment inspection, and are configured to obtain images of alignment targets or other identifiable features associated with the vehicle for communication to the processing unit. Correspondingly, the processing unit is configured with one or more software applications, at least one of which is adapted to facilitate the alignment of vehicle wheels which generally consist of a rim and an associated tire, using input received from the imaging sensors.
The machine-vision imaging sensors are traditionally part of a camera system or imaging system configured to view optical targets within associated fields of view to obtain images thereof for processing by the software applications in the console. Commonly, the observed optical targets incorporate highly accurate patterns that have known control features. The three-dimensional positions and relationships of the features in the images are determined, and the orientation of the wheels or other vehicle components to which the optical targets are attached are calculated by well known algorithms. Exemplary configurations for the high-accuracy optical targets are described in U.S. Pat. No. 6,064,750 to January, and in U.S. Pat. No. 6,134,792 to January. Each exemplary optical target consists of a target face, on which are disposed identifiable optical elements, a precision flat base, and a mounting shaft adapted for attachment to a separate clamping assembly secured to the vehicle or vehicle wheel assembly.
The conventional configuration for an optical target is precisely engineered with high-contrast optical elements such as circles, squares, or triangles. The accuracy of such conventionally configured optical targets is dependant upon how well the high contrast edges of the optical target elements can be located in an image produced by the imaging components of the wheel alignment system. For the best accuracy, the individual optical elements must be large enough to have relatively long straight or curved boundaries, and they must be separated far enough to prevent the individual optical target elements from appearing to fuse into a single object when reduced edge sharpness causes two or more optical target elements to bleed into the same pixel in the imaging system. These factors combine to limit the number of individual image pixels generated by the imaging system whose values are utilized to calculate a position and orientation of a conventionally configured optical target.
Each image of a conventional high-contrast optical target acquired by the optical imaging vehicle wheel alignment system is processed to identify a number of reference points in the image. Either the computer or the imaging system is configured to mathematically manipulate the positional relationships of the observed reference points, as identified in an image, to match them with a set of predetermined positional relationships based on the known parameters of the conventional high-contrast optical target. Once the relationship between the observed positional relationships and the predetermined positional relationships is identified for the reference points, the position and orientation in three-dimensional space of the target (and an associated vehicle wheel) relative to the position and orientation of the imaging system is identified, from which one or more vehicle wheel alignment angles can be identified. Accordingly, for an optical imaging vehicle wheel alignment system to function, it is necessary for the system to be capable of extracting a set of control or reference points from acquired images.
To further facilitate the operation of a machine vision vehicle wheel alignment system, the separate optical targets are secured to the vehicle wheels with precision wheel adaptors configured to clamp onto the vehicle wheel edges and to position a mounting point for the optical target substantially coaxial with the wheel rim's axis of rotation. The traditional precision wheel adaptors typically include a set of claws or feet adapted to secure the wheel adaptor to the vehicle wheel assembly by engaging the lip or rim of the wheel rim at the tire junction. A centering mechanism on the wheel adaptor ensures that the claws or feet of the wheel adaptor are adjusted in a symmetrical manner to maintain the mounting point for the optical target in a determined centered configuration in relation to the axial center of the wheel rim.
Some variations of traditional wheel adaptors, such as the Tire Clamp Adaptor Model No. 20-1789-1 from Hunter Engineering Co., and those shown in U.S. Pat. No. 5,987,761 to Ohnesorge and U.S. Pat. No. 6,131,293 to Maioli et al. further utilize a set of gripping arms adapted to engage tire surfaces in conjunction with a set of contact supports and centering mechanisms for symmetrically engaging the circumferential lip of the wheel rim and securing the wheel adaptors in an axially centered position on the vehicle wheel assembly.
Other vehicle-specific wheel adaptors, such as those for use with Mercedes Benz and BMW automobiles, are configured with a set of pins which are designed to pass through the wheel assembly, and to contact predetermined surfaces on the vehicle wheel hubs, positioning the vehicle-specific wheel adaptor in a predetermined axially centered location about the wheel assembly. These vehicle specific wheel adaptors are then held in place by means of tire clamps or spring mechanisms which grip to the tire tread surfaces.
Traditional wheel adaptors that will universally adapt to the wide range of wheel sizes on the market today are difficult to design and costly to build. Many times additional parts are required, such as extenders, in order to allow the adaptor to work with wheels that are very small or very large which also adds additional cost and complication to the adaptor system. Additionally, traditional adaptors have to provide a substantial amount of clamping force in order to hold the weight of the target or sensor on the wheel assembly. This clamping force can scratch or dent the wheel assembly where it is attached. This is very undesirable especially when the wheel assembly is a very costly aftermarket wheel.
Accordingly, it would be advantageous to provide a machine vision vehicle service system, such as a wheel alignment system, with an optical target assembly which incorporates both an optical target and a simplified adaptor for attachment the optical target assembly to a vehicle wheel, and which does not require a determined precision mounting on the vehicle wheel assembly in relation to the wheel axis of rotation.
It would be further advantageous to provide a machine vision vehicle wheel alignment system with a mechanically simplified optical target assembly which is light weight, dimensionally stable, less abrasive to the wheel rim surfaces, and which does not require precision construction.